Circuits using transistors that perform switching operation, such as converters, inverters, and power factor correction circuits become a generation source of noise since the switching operation suddenly changes a voltage. A generation amount of noise depends on a slew rate of a drain voltage of the transistor. In order to inhibit noise, it is common to insert a resistor in series into a gate terminal of the transistor, and decrease a change of a gate voltage to suppress the slew rate of the drain voltage.
However, there is a problem that reducing the slew rate of the drain voltage increases switching loss. The slew rate also depends on a load current, an element temperature, element variation, and the like. Therefore, when a value of gate resistance is set such that noise does not exceed a predetermined value under any operating conditions, the slew rate becomes significantly smaller than the predetermined value in many periods, causing a state where switching loss occurs more than necessary.
In order to solve such a problem, it is sufficient to observe the slew rate of the drain voltage, that is, a transition time, and adjust a gate drive signal such that a change amount becomes a predetermined value. For example, a method has been proposed in which a rise time of a drain voltage is detected, and a difference between the detected rise time and a target time is fed back to a drive circuit of an FET. However, no specific method for generating a gate drive signal of the FET has been proposed. Therefore, it is not possible to adjust the slew rate of the drain voltage to the target value.